1. Field of the Invention
This invention is in the field of a power supply for electrochemical machining of the type employing a nozzle composed of insulating material which directs a stream of cathodically charged electrolyte at the workpiece to drill holes of controlled diameter through the workpiece.
2. Description of the Prior Art
There have been a number of electrochemical machining methods described and developed in the past. Electrochemical machining involves the use of a tool in the form of a nozzle which is made cathodic and which cooperates with an anodic workpiece through an electrolyte which contacts both the tool and the workpiece. Upon flow of electric current through the electrolyte, material of the workpiece is removed or deplated. These processes have been used to a large extent in drilling small diameter holes in metal which are difficult to drill mechanically.
One of the more recently developed processes employing electrochemical machining is the impingement process which is sometimes known as "Electrostream" (a trademark of General Electric Corp.) drilling. This process has been used to drill extremely small holes in tough metals such as nickel base superalloys. With this process, it is possible to drill holes having a diameter on the order of 0.005 to 0.020 inch. The impingement process normally makes use of a glass tube nozzle with an electrode behind the tip of the nozzle. The impingement process is operated under conditions of relatively high resistance and high voltage, typically on the order of 300 to 600 volts DC. Either a salt or an acid electrolyte is used. Electrolyte pressures in this type of process are typically on the order of 20 to 100 pounds per square inch.
It has been found difficult, however, in conventional electromachining operations of the impingement type to drill holes which have a uniform diameter throughout their axial length. Experience has indicated that the holes produced generally have tapered sides in the portion of the hole nearest to the electrode nozzle, and the desired diameter of the hole appears only at the vicinity of the hole farthest from the electrode. This condition, known as "washout" is particularly troublesome where a plurality of holes are to be drilled in closely spaced relation since the washout from adjoining holes can overlap resulting in an undesired depletion of metal in the surface of the workpiece between the holes.
The impingement process as it is presently practiced also suffers from the disadvantage that there is excessive wear on the tip of the insulating nozzle, requiring replacement of nozzles at relatively frequent intervals. While the cause for such wear has not been precisely determined, it is believed that superheated steam is generated under the conditions of sustained voltage application and that this steam attacks the glass electrode resulting in erosion by leaching of the glass composition.